This invention relates to novel color developers useful in recording materials, especially acid-base color formation type recording materials such as heat sensitive recording materials, carbonless pressure sensitive recording materials, etc. which utilize color forming reaction of normally colorless or palely colored basic leuco dyes with color developers which are acid substances.
Color developers which have been popular for this use include phenolic organic materials represented by 4,4'-isopropylidene diphenol (so-called bisphenol A), benzyl p-hydroxybenzoate, novolak type phenolic resins, zinc salts of salicylic acid derivatives, etc. inorganic solid acids represented by activated clay, silica-magnesia solid acids, etc. However, these materials have meits and demerits in practical performances and suffer problems to be solved.
Bisphenol A and benzyl p-hydroxybenzoate among the above known color developers have often been used as color developers for heat sensitive recording materials. Bisphenol A has a problem of difficulty in obtaining heat sensitive recording materials of high sensitivity. On the other hand, benzyl p-hydroxybenzoate is excellent in attaining high sensitivity, but there sometimes occurs the phenomenon of production of white fine crystals in color image areas formed by heat impression with lapse of time, so-called blooming phenomenon. This is a kind of undesired image deterioration.
Hitherto, organic or inorganic solid acids such as novolak type phenolic resins, zinc salts of salicylic acid derivatives, activated clay, silica-magnesia, etc. have been used in pressure sensitive recording materials. However, particularly inorganic solid acids are insufficient in fastness of color images formed, especially light-resistance.